1. Field of the Invention
The present invention generally pertains to surfactants, more specifically, to surfactants useful for preparing aqueous dispersions of tackifier resins, where the resins may be blended with latex to provide waterborne adhesives.
2. Description of the Related Art
The ongoing development of improved waterborne (i.e., water-dispersed) adhesives to replace solvent-based adhesives has largely been motivated by government regulation of solvents, stemming from the perceived health and environmental hazards thereof. As a result, waterborne adhesives have become extensively used to adhere various substrates together. Such adhesives often comprise both a high molecular weight elastomeric latex (or mixture of latexes, such as acrylic and styrene-butadiene rubber latex), and at least one water-dispersed tackifier resin. It is possible to prepare a functional waterborne adhesive without a tackifier resin. However, inclusion of the latter is generally required to economically produce an adhesive exhibiting an adhesion to various substrates that allows for commercial applicability of the adhesive.
A significant portion of commercial waterborne adhesives are pressure sensitive adhesives (“PSAs”). The latter are used, for example, for adhesive tapes; to adhere labels to the cellulosic, polyolefinic and polar-polymeric substrates of bottles and other packaging containers; and to adhere cabinet and wall decorations and the like.
While PSAs generally exhibit good adhesion to a variety of substrates, their adhesive performance is limited. For example, the adhesion of PSAs to substrates is generally inferior to that of their solvent-based counterparts. This deficiency may be attributed, at least in part, to the surfactant used to disperse the tackifier resin or prepare the latex or blend of latexes used for the PSA. Surfactants inherently plasticize polymer-tackifier resin blends, which reduces adhesive shear. Also, surfactants often impede adhesion by migrating to the substrate interface, thereby interfering with the ability of the adhesive to stick thereto.
Waterborne PSAs are also characterized by a limited shelf life, by a limited ability to wet adhesive substrates (i.e., the substrates used to make adhesive tapes, labels, and the like), and by a mechanical stability that is less than ideal for high-shear and high-speed applications. Extended shelf life is generally desirable. Exceptional adhesive substrate wetting and superior mechanical stability are important for high-speed coating of an adhesive substrate by an adhesive. High-speed coaters, such as gravure, reverse gravure and slot die coaters, are now commonly used to produce lower coat weights and allow for increased throughputs.
Again, the surfactant used in preparing the tackifier resin dispersion is a limiting factor. Use of nonylphenol-based surfactants has yielded tackifier resin dispersions and, therefrom, PSAs that exhibit substrate wetting and mechanical stability that is compatible with the above-mentioned high-speed coating methods. However, governmental regulatory activity has been increasingly directed to nonylphenols and their derivatives to reduce or eliminate their presence in the marketplace, as well as in chemical plant aqueous effluents.
Methods for the self-emulsification of resins have been described. See, e.g., U.S. Pat. Nos. 4,670,504 and 5,268,399, and Great Britain Patent Nos. 2,273,294 and 2,288,179. In these methods, a resin is treated so that hydrophilic groups are added to a small portion of the bulk resin. That is, a small amount of hydrophilic groups are added to a resin, so that hydrophilic groups are introduced to a small fraction of the bulk resin molecules. The resin molecules containing these hydrophilic groups then act as a surfactant when water, and optionally some co-solvent, are added to the resin/modified resin mixture.
Accordingly, there remains a need in the art for surfactants that are useful for preparing tackifiers that, when blended with latex; yield waterborne adhesives, in particular, PSAs, having improved adhesion, improved adhesive substrate wettability, greater mechanical strength for high-speed coating operations, and an improved shelf-life. The present invention fulfills these needs and provides further related advantages as described herein.